Wireless terminal such as cellular phone comprises RF (Radio Frequency) signal line. The RF signal line is normally made of coaxial cable. Such RF signal line made of coaxial cable makes the inner space of the wireless terminal narrower so that flexible circuit board generally applied to the wireless terminal nowadays.
Normally, a flexible circuit board reduces line loss and reduces reflection loss of a signal, thereby securing a necessary signal condition. Reducing the line loss is possible by increasing the area of the signal line or by increasing the thickness of the signal line. The reduction of the reflection loss is also possible by shielding an external signal from the outside.
Flexible circuit board has optimum impedance of about 33Ω at the signal transmitter, and optimum impedance of about 75Ω at the signal receiver. Considering both the transmitter and receiver, it is ordinary that flexible circuit board should be designed to have characteristic impedance of about 50Ω.
In the case that external environmental signals are introduced from other outside components, the characteristic impedance would be deviated from the normal level of 50Ω, so that the signal transmission efficiency might be aggravated.
Therefore, flexible circuit board should be applied at the reasonably separated location from other components so as to prevent unwanted impedance variation. In this case, however, the inner space of the terminal, which is the main merit and advantage of flexible circuit board, may be narrower.
Flexible circuit board may be placed near other components by the impedance matching. But, in this case, the shape of flexible circuit board should be changed as well, according to the variation of shape or location of at least one component among its main board, sub board, battery or so. Therefore, it is necessary that the flexible circuit board can be installed freely inside of the wireless terminal without effect of other components thereof.
The background described herein will be understood only for the purposes of understanding of background of the invention, so that said background should not be accepted the prior art of the invention in the view of the person having ordinary skilled in the art.